There is a general interest in servo-controlled piston/cylinder pumps. For example, SCIREQ's flagship product, the flexiVent™, is essentially a computer-controlled piston pump that is customized for use as a mechanical ventilator and measuring device of pulmonary mechanics for pre-clinical research.
Servo-controlled piston pumps typically consist of i) an actuator, ii) a piston/cylinder assembly, iii) a position/displacement measurement device, and iv) gears, rods or other means of connecting the other components together in order to transmit force from one component to the other. While in some cases, the actuator and the position measurement are somewhat integrated, e.g. when a stepper motor is used, there is no known solution in the public domain that combines the actuator and the cylinder into a single device in such a way that there is no need for connecting rods or gears whatsoever.
The arrangements described above have a number of disadvantages. First, their size cannot easily be minimized because space must be provided for the connecting rods. For example, a setup based on a DC linear actuator can easily require an overall length that is greater than four times its actual stroke length, Furthermore, the connecting gears and rods can pose problems and/or reduce performance due to weight, insufficient stiffness, poor alignment, play in joints, friction and dynamic properties, Finally, the multiplicity of components adds to the overall system cost.
Thus, there is still presently a need for development of a self-contained linear actuator/cylinder that would integrate a linear actuator, a cylinder to displace gases or liquids and means to measure position into a single device that i) does not require any connecting rods or gears; and ii) fits into a smaller envelope that conventional actuator/cylinder assemblies with comparable stroke volume.
Additionally, there is a need for an improved integrated device for spirometry and forced oscillation pulmonary mechanics. In pulmonary medicine, the breathing pattern of a patient is often quantitatively assessed by recording the airflow at the patient's mouth and/or nose and deriving a number of parameters such as tidal volume and breathing frequency. Often, the patient is also asked to perform specific manoeuvres such as a deep inflation followed by a hard expiration in order to measure the forced expired volume in one second (FEV1) and the forced vital capacity (FVC). This process is commonly referred to as “spirometry”.
In recent years, a technique known as the Forced Oscillation Technique (FOT) has emerged as a possible alternative to spirometry. Briefly, the FOT measures the input impedance of the respiratory system, typically in the frequency band from sub-acoustic frequencies to roughly 50 Hz, by imposing small amplitude waveforms onto the subject's airway opening. The resulting flows and pressure swings are recorded and used to calculate the real and imaginary parts of the input impedance. Devices to obtain FOT measurements in humans are typically based on large loudspeakers that are connected to the subject's airway opening via long tubing. A side port with a calibrated resistance and/or a bias flow ensures that the patients do not re-breathe their own expired air. The resulting airflow is estimated or measured using pneumotachographs.
The FOT has the advantages that it requires less patient cooperation, that it offers detailed information and that the parameters it measures relate directly to the physics of the lungs. However, current FOT devices are limited because of their large size, poor low frequency performance and poor coupling between the patient and the device.
Consequently, there is still presently a need for an “Oscillation Spirometer” (OS) that i) can act both as a spirometer and to obtain FOT measurements; ii) is compact and light enough to be portable and permit handheld operation; iii) places the FOT waveform generator in close proximity of the airway opening to permit good coupling between patient and device; iv) offers good performance down to ultra-low frequencies, and v) can possibly be integrated into a mechanical ventilator circuit.